The present invention relates to a method for efficient evaluation of various materials and determination/improvement of the conditions for their production, using measured values of the phase velocity (hereinafter referred to simply as velocity) of surface or bulk acoustic waves.
Under present circumstances, the evaluation of materials (crystals, in particular) widely used in electronic device industries is made, in general, by use of the lattice constant measured by X-ray diffractometry and the Curie temperature by differential thermal analysis or the like. In particular, surface acoustic wave (SAW) device materials are mostly evaluated empirically using parameters (lattice constant and Curie temperature) in place of the velocities of acoustic waves.
At the laboratory level, it is common practice to evaluate materials by use of the phase velocities of leaky surface acoustic waves (LSAWs) measured by line-focus-beam (LFB) ultrasonic microscopy [Literature 1]. LSAW is an acoustic wave that propagates at the boundary between a fluid medium (water couplant) and the specimen surface, and its propagation characteristic is specific to every material. The main factor that dominates the homogeneity of crystals is the distribution of their chemical composition. A change in the chemical composition is detected as a change in the LSAW velocity to evaluate the crystals. In this case, the relationship (a calibration line) between the chemical composition and the LSAW velocity needs to be premeasured experimentally. This involves: the preparation of a specimen of the same cut plane as that of every substrate to be evaluated; measurements of the LSAW velocity in a particular propagation direction and the Curie temperature of the specimen by differential thermal analysis; and the determination of the above-mentioned relationship between the LSAW velocity and the chemical composition based on the measured values of the LSAW velocity and Curie temperature. This procedure is inefficient because it must be performed prior to the evaluation of each substrate for different crystal planes and different propagation directions.
In the case of SAW device materials, in particular, evaluation by the SAW velocity is a direct evaluation and hence is important or preferable from the practical point of view since the SAW devices utilize the propagation characteristics of SAWs that propagate on the substrate surface with no water loaded thereon (a free surface). Theoretically, however, LFB ultrasonic microscopy is limited specifically to the measurement of the SAW velocity in the propagation mode (Rayleigh type) in which to excite the SAWs on the water-loaded substrate surface; hence, it is difficult to directly measure the SAW velocity (especially, a shear horizontal wave (SH wave) type (hereinafter referred to simply as SH-type) SAW). At present, the SAW velocity could be measured, for example, by directly exciting SAWs on the substrate surface with electrodes formed thereon, but few material manufacturers adopt such a direct evaluation method.
In the material evaluation by LFB ultrasonic microscopy, the amount of change in the LSAW velocity due to a change in the chemical composition of the material differs for different substrate surfaces and different propagation directions, and a calibration line that represents the relationship between the chemical composition and the LSAW velocity must be prepared experimentally for each substrate and for each propagation direction; this is time-consuming and laborious. Further, in order that the change in the chemical composition detected as the LSAW velocity may be evaluated as the SAW velocity, it is necessary to experimentally obtain the relationship between the SAW and the LSAW velocity by measuring the SAW velocity for a substrate with electrodes formed on its surface, but this method is inefficient.
It is therefore an object of the present invention to provide a method for efficient material evaluation by preparing the calibration line of the acoustic velocity with respect to the chemical composition without the need for experimentally preparing it for all possible combinations of substrate crystal planes and directions and modes of propagation.
Another object of the present invention is to provide a material evaluation method that permits direct numerical calculation of the relationship of velocity between acoustic waves in various modes of propagation (for example, between the SAW and the LSAW and between longitudinal and shear waves) for an arbitrary chemical composition without involving any particular experimental procedures.
Still another object of the present invention is to provide a material evaluation method that allows efficient evaluation of acoustic characteristics, permitting direct evaluation of materials by use of the acoustic wave velocity measured therefor with high accuracy, in place of the Curie temperature and lattice constant.
According to the present invention, desired independent, acoustical physical constants of materials (elastic, piezoelectric and dielectric constants and density for a piezoelectric material, and elastic constants and density for a non-piezoelectric material) are predetermined as a function of the chemical composition. This enables numerical calculation of the acoustic velocity for a desired combination of the substrate cut plane and the direction and mode of propagation, permitting efficient evaluation of materials and determination/improvement of the conditions for their production by acoustic velocity measurements.